1. Field of the Invention
The present invention relates generally to the field of flat panel displays. More specifically, the present invention discloses a technique using a light re-distributor (LRD) for increasing the brightness of a display at viewing envelopes/angles that are away from the direction that is perpendicular to the display panel (display normal).
2. Statement of the Problem
Normally, displays are brightest when viewed along the display normal direction and brightness decreases with the increase of viewing angles. In the case of an LCD, a diffuser behind the display panel roughly defines the angle distribution of the illumination. A good diffuser gives approximately Lambertian distribution, that is, the brightness is proportional to cos2(θ), where θ is the angle between the viewing direction and the display normal. The brightness of an actual display may decrease faster than the Lambertian distribution because of the reflection of the surfaces of the display panel in front of the diffuser. The luminance at the display normal can be a couple of times larger than the luminance at a large viewing angle, say 60° from the display normal.
In certain situations, such as in the cockpit of a transport aircraft, the pilot's and the co-pilot's primary viewing directions can be at very large angles, say 50°, from left and right. The primary viewing envelopes are then centered on the −50° and +50° degrees horizontal and the viewing envelope centered on the display normal is secondary. In other cases, such as displays in other types of aircrafts and automobiles, the primary viewing directions can also be away from the display normal. In all these cases, it is desirable to increase the brightness at the primary viewing envelopes. In some cases, it is even more desirable for the display to be brighter at primary viewing envelopes than at the secondary. A technique to boost the brightness of these primary viewing envelopes is of importance.
We disclose a method of increase the brightness of the primary viewing envelopes away from display normal using a light re-distributor.
3. Prior Art
A scheme of redirecting light of display has been proposed in U.S. Pat. No. 5,303,322. A so-called image redirect film redirects the light to a certain angle of viewing [1]. FIG. 1 is a description of the prior art. Compare to the method disclosed in this application, the prior art has the following limitations: 1). It redirects light to one direction or envelope but not to multiple directions or envelopes. 2). It can only be achieved with a special prismatic film and not with other optical elements that can be more cost effective. 3). It shows an artifact that when close to the cut-off angle, the brightness changes dramatically if the viewer moves his head slightly. This artifact may also cause luminance non-uniformity problem if the display is large or if a few small displays is put side by side so that the total display area extends a certain viewing angle range to a fixed eye position.
4. Solution to the Problem
The present invention provides a flat panel display that uses a light re-distributor (LRD) to re-distribute light between the primary and secondary viewing envelopes of a display. This approach successfully addresses the problem of a conventional display's being much brighter at the display normal than at larger viewing angles and boosts up the brightness of the display at larger viewing angles. The present invention has the following advantages:                (1) Increase the brightness of the display at the primary viewing envelopes that are away from the display normal. For displays primarily viewed from large viewing angles, or viewed from a wide range of angles (such as TV set), this invention is valuable.        (2) Reduce power consumption requirement because the required brightness at primary envelopes can be achieved with a less bright backlight so that it uses less power. And, running at a lower backlight power, the backlight, and the display to a certain degree, will have a longer lifetime.        (3) No disturbance to the overall lighting of the environment where the viewer is in so that the viewing of other displays or other information will not be affected. Without the use of this invention, when the display achieves the brightness requirement at the primary viewing envelopes, the brightness at the secondary viewing envelope, i.e., around the display normal, can be so bright that it interferes with the overall lighting.        (4) Cosmetic defects are less visible. While boost the brightness at larger viewing angles, this invention reduces the visibility of small manufacturing cosmetic defects in the display panel by reducing the intensity of backlight illumination at the display normal. Defects are much less visible when backlight is less intense. This invention redirect a portion of the light otherwise coming out of the display normal to larger viewing angles so that they are less visible at the display normal.        (5) Provides multiple ways of realizing the desired effects. It is of low cost, easy to manufacture because of the fact that it can be made of different kinds of and commercially available materials.        